GTK Signals
Callback functions invoked when performing actions on GTK widgets are called 'signals' in GTK-speak.
Each GTK widget supports a predefined set of signals. For example, a top-level window supports the 'destroy' signal, invoked when the user kills the application via the window manager.
Other widgets will have other types of signals - e.g. a button will have a 'clicked' signal. A combo box will have a 'selection' signal.